It is well-established in automotive internal combustion engine controls that conventional engine exhaust gas catalytic treatment devices operate at high efficiency when the engine air/fuel ratio is controlled substantially at the stoichiometric ratio, as illustrated by the curves 100, 102, and 104 of FIG. 1, representing the efficiency in treating the exhaust gas constituent elements of hydrocarbons HC, carbon monoxide CO and oxides of nitrogen NOx, respectively, as a function of engine air/fuel ratio. Feedback signals indicating air/fuel ratio control performance are provided by engine exhaust gas oxygen sensors which have a characteristic output "S" voltage curve, such as curve 110 of FIG. 2. Actual engine air/fuel ratio information may be provided by monitoring the output signal of an oxygen sensor positioned in the engine exhaust gas path upstream, according to the normal direction of exhaust gas flow, from the catalytic treatment device. The performance of the overall engine emissions control system including that of the air/fuel ratio control system and the exhaust gas treatment system may be determined by monitoring the output of an oxygen sensor positioned in the exhaust gas path downstream, according to the normal direction of exhaust gas flow, from the catalytic treatment device.
When the downstream oxygen sensor output signal is within a predetermined voltage range defined by lower voltage Vl and upper voltage Vr, which may be approximately 200 and 600 millivolts, respectively, for a typical zirconium oxide sensor as illustrated by curve 110 of FIG. 2, a healthy catalytic treatment device should operate efficiently. Any signal deviation outside the predetermined voltage range should be rapidly driven back into the range through the activity of the closed-loop engine air/fuel ratio controller, and through the oxygen storage and release activity of the conventional catalytic treatment device. Signal 120 of FIG. 3 illustrates a typical output signal pattern for a downstream oxygen sensor in a properly operating engine and emissions control system with a high catalytic treatment efficiency, indicated by only temporary excursions of the downstream sensor signal VO2 into saturation, indicated by a signal VO2 being below Vl or above Vr.
Downstream oxygen sensor voltage signal deviations outside the predetermined voltage range that persist for a significant amount of time indicate a system performance problem that may likely lead to a significant reduction in exhaust gas aftertreatment efficiency which, if left unremedied, may lead to substantial increase in vehicle emissions. Signal 122 of FIG. 4 illustrates a typical downstream oxygen sensor signal with a rich bias which may indicate an engine or emissions control system fault condition, such as may be caused by a heavily loaded evaporative canister, indicated by too significant an amount of time above the threshold signal voltage Vr, and signal 124 of FIG. 5 illustrates a potential fault condition corresponding to a lean bias of the downstream sensor, such as may be caused by a system air leak, indicated by too significant an amount of time below the threshold signal voltage Vl.
Proposed diagnostics to detect performance deterioration in specific engine and exhaust system components are typically complex, throughput intensive, and expensive to incorporate, making them poorly suited to applications requiring system level diagnostics with even reasonable controller throughput and cost constraints, and making them difficult to install, calibrate and maintain.
It would therefore be desirable to develop a simple, inexpensive, yet reliable engine control system diagnostic, especially to diagnose whether the engine air/fuel ratio controller is operating in a manner supporting efficient operation of the catalytic treatment device.